The present invention relates generally to speakerphones, and more particularly to an improved speakerphone using digitally compressed audio to control voice path gain that can advantageously be utilized in mobile radiotelephones.
In both radio and landline telephone systems, a user communicates by means of a handset that includes a speaker at one end which is placed close to the user's ear and a microphone at the other end which is held close to the user's mouth. Thus, the user has only one free hand since the other must be used to hold the telephone handset. In order to provide a greater degree of freedom to the user, speakerphones have been developed for use in landline telephone systems. Conventional speakerphones, such as that described in U.S. Pat. No. 3,751,602, are coupled to the telephone line and when enabled allow the user to freely move about while still communicating with another party. Such speakerphones typically compare the volume of the two parties and select the party speaking the loudest. Furthermore, conventional speakerphones always revert to the listen mode a short time interval after the hands-free party stops talking. However, conventional speakerphones suffer from a number of problems including inability to accommodate high ambient noise environments and breakup due to feedback from the speaker to the microphone when the speaker amplification has been turned up. Although these conventional speakerphones work reasonably well when utilized with landline telephones in relatively quiet surroundings, they do not operate correctly when used in noisy environments and especially when used with a radio due to the high degree of ambient noise encountered in vehicles. For example, a high level of ambient noise may cause such speakerphones to always select the hands-free party.
The problem of accommodating a high degree of ambient noise has been addressed in the speakerphones described in U.S. Pat. Nos. 4,378,603 and 4,400,584 by selecting only one of the parties while muting the other party, and switching between parties using the output of audio signal detectors that detect the presence of audio signals only when the audio signals exceed the background noise by a predetermined amount. Such speakerphones require dedicated special-purpose circuitry for detecting audio signals and producing various time constants, that is usually in a custom integrated circuit and can not be adjusted without changing associated resistors and capacitors. Accordingly, there is a need for an improved speakerphone that uses digitally compressed audio to control voice path gain.